Tandem photovoltaic cells consisting of at least two absorbers with different band gaps allow broader spectrum light harvesting and superior photovoltaic conversion efficiency as compared to single-junction solar cells. Tandem photovoltaic cells are often oriented with one solar cell on top of another. For optimal performance, the bandgap of the absorber in the top solar cell should be higher than the bandgap of the absorber in the bottom solar cell.
Two commonly employed types of tandem device are two-terminal devices and four-terminal devices. Two-terminal tandem devices contain one electrode on top and one electrode on the bottom, with a tunnel junction between the top and bottom solar cells of the device. Four-terminal tandem devices contain independent devices stacked on top of each other, wherein each independent device has its own top and bottom electrodes. Two-terminal tandem devices are more challenging to fabricate than four-terminal tandem devices because two-terminal tandem devices require current-matching between the top and bottom solar cells. Further, care must be taken during fabrication of two-terminal tandem devices to not damage the bottom solar cell during processing of the top solar cell. While having less strict current-matching and processing constraints than two-terminal devices, four-terminal devices nonetheless suffer from significant resistance and optical losses due to their need for multiple transparent conductive contacts and reflection losses associated with the additional substrates and layers.
Chalcogenide-based solar cells such as CuInSe2 (abbreviated as “CIS”), Cu(In,Ga)(S,Se) (abbreviated as “CIGS”), and Cu2ZnSn(S,Se)4 (abbreviated as “CZT(S,Se)”) have achieved their highest efficiency at relatively low bandgap (approximately 1.15 electron volts (eV)). The use of chalcogenide-based solar cells in a tandem device architecture however presents some notable challenges. For instance, for maximum performance, chalcogenide-based solar cells require very high processing temperatures of the absorber layer (above 450 degrees Celsius (° C.)). Thus, chalcogenide-based solar cells often cannot be used as the top solar cell in a tandem device since these high temperatures would degrade the bottom solar cell. Further, the low band gap of a chalcogenide absorber makes chalcogenide solar cells not well-suited for use in a top cell.
Once formed, chalcogenide-based solar cells employ a p-n junction that significantly deteriorates at temperatures above approximately 200° C. Thus, when a chalcogenide-based solar cell is used as the bottom cell, processing temperatures for the top cell must be kept below about 200° C. to maintain the p-n junction in the bottom cell. This requirement can be a challenge meet when using conventional solar devices for the top solar cell.
Therefore, techniques for integrating chalcogenide-based solar cell designs into a tandem photovoltaic device architecture would be desirable.